This invention relates to expandable olefin polymer compositions and processes, and more particularly to expandable modified olefin polymer compositions having dimensional stability utilizing low cost isobutane as a primary blowing agent.
It is well known to prepare olefin polymer foams by heat plastifying a normally solid olefin polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel and thereafter extruding the gel into a zone of lower pressure and temperature to activate the blowing agent and expand and cool the gel to form the desired solid olefin foam product.
A problem frequently encountered is that of preventing an unacceptable degree of shrinkage of partially cured foam during the aging or curing period following manufacture. During the aging or curing period the blowing agent employed gradually diffuses out of the cells in the foam product and air gradually diffuses into the cells in place thereof. Until quite recently, it was believed that only one volatile hydrocarbon blowing agent, namely 1,2-dichlorotetrafluoroethane, was capable of providing sufficient dimensional stability during the curing period to permit the commercially viable manufacture of low density (e.g., 1 to 6 pounds per cubic foot (16 to 96 kg/m.sup.3) foams of ethylenic polymer resins. That is, only dichlorotetrafluoroethane was believed to diffuse out of the foam cells slowly enough to prevent cell wall collapse while air was slowly diffusing into the cells.
More recently, permeability modifiers or stability control agents have been developed for incorporation into the polyolefin in an attempt to slow the diffusion of volatile hydrocarbon blowing agents out of polyolefin foam cells. The objective of these permeability modifiers is to render the foams more dimensionally stable to a wider variety of volatile hydrocarbon blowing agents. For purposes of this invention, the terms "permeability modifier" and "stability control agent" will be used interchangeably and will refer to compositions incorporated into the polyolefin to slow diffusion of volatile hydrocarbon blowing agents from the foam cell walls. For example, Watanabe et al, U.S. Pat. No. 4,214,054, teaches the production of polyolefin foams utilizing volatile hydrocarbon blowing agents. Permeability modifiers such as saturated higher fatty acid amides, saturated higher aliphatic amines, and esters of saturated higher fatty acids are incorporated into the polyolefin composition prior to expansion.
Park, U.S. Pat. No. 4,331,779, also teaches ethylenic polymer foams having improved dimensional stability and teaches the use of a copolymer of ethylene and an unsaturated carboxylic acid as a stability control agent. Park, U.S. Pat. No. 4,347,329, teaches the use of a fatty acid amide such as stearamide for use in polyolefin foams as a stability control agent. Park, U.S. Pat. No. 4,394,510, further teaches the use of fatty acid amide stability modifier agents to produce polyolefin foams having improved elevated temperature dimensional stability.
The use of such permeability modifiers permits the use of a wider variety of volatile hydrocarbon blowing agents. However, in many cases, the more inexpensive volatile hydrocarbon blowing agents such as butane can only be utilized in small amounts in conjunction with other more expensive chloro- or fluorocarbons. In instances where butane has been used alone as the blowing agent in modified polyolefin foams, the foams exhibited maximum shrinkages defined as (1-the ratio of the volume of the foam on the day it is at a minimum to the volume of the foam immediately after expansion).times.100 of between 10 and 20%. See, for example, examples 21, 24, and 37 at Table 7 of Watanabe et al, U.S. Pat. No. 4,214,054.
Accordingly, the need still exists in the art for low cost volatile hydrocarbon blowing agents which can be used to expand olefin polymers and yet exhibit a high degree of dimensional stability with minimal shrinkage during aging or curing of the polymer foams.